This slip-up actually occurs a couple of times throughout the movie.

While pitching the idea of "Spectacular Spectacular" to the Duke, Christian explains that the Sitar is magical and can only speak the truth.

While rehearsing for "Spectacular Spectacular" Christian says to Toulouse "Toulouse, so, the magical sitar player falls from the roof and says 'the greatest thing you'll ever learn is just to love and be loved in return'".

He also says it prior to that when they're rehearsing in his humble abode. The magical sitar is the one who is meant to fall from the roof and say this line, not the magical sitar player.

It seems like a fairly big part of the movie and I assumed it would have been picked up on.